All normal retroviral particles contain a dimer of genomic RNA. The physical structure of the dimer, and of the linkage between the two monomers, is not understood. We reported some years ago that the viral nucleocapsid protein alters the linkage between the monomers when it is released from Gag during viral maturation. This stabilization of the dimeric linkage results from the nucleic acid chaperone activity of nucleocapsid. We have continued to probe the structure of viral RNA within authentic virus particles. We have recently analyzed the RNA by cleaving it at specific sites using RNase H and short oligodeoxynucleotides complementary to specific sequences in the RNA. The results of these experiments, for the first time, localized the most stable linkage between the monomers to the region near their 5'ends, in both immature and mature murine leukemia virus (MLV) particles. The experiments also revealed the existence of additional, less stable linkages between the monomers. Retroviral nucleocapsid proteins are highly active nucleic acid chaperones. The mechanism of this crucial activity is not well understood. We have analyzed the binding of HIV-1 nucleocapsid to a very short (8-base) oligodeoxynucleotide in great detail, using several biophysical techniques. We found that a single nucleocapsid molecule can bind simultaneously to two nucleic acid molecules;conversely, a single nucleic acid molecule can bind two nucleocapsid molecules. It seems likely that the ability of the protein to interact with more than one nucleic acid molecule is a critical element in its nucleic acid chaperone activity. We are currently analyzing packaging signals in both HIV-1 and MLV viral RNAs, using both genetic and structural methods. [Corresponds to Rein Project 2 in the April 2007 site visit report of the HIV Drug Resistance Program]